Note: Descriptions are shown in the official language in which they were submitted.
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FUEL CELL SYSTEM
The present invention relates to fuel cells and in particular to proton-
exchange
membrane type fuel cells in which hydrogen is supplied to the anode side of
the fuel cell,
oxygen is supplied to the cathode side of the fuel cell and water by-product
is produced
at and removed from the cathode side of the fuel cell.
Such fuel cells typically comprise a proton exchange membrane (PEM) sandwiched
between two porous electrodes, together comprising a membrane-electrode
assembly
(MEA). The MEA itself is conventionally sandwiched between: (i) a cathode
diffusion
structure having a first face adjacent to the cathode face of the MEA and (ii)
an anode
diffusion structure having a first face adjacent the anode face of the MEA.
The second
face of the anode diffusion structure contacts an anode fluid flow field plate
for current
collection and for distributing hydrogen to the second face of the anode
diffusion
structure. The second face of the cathode diffusion structure contacts a
cathode fluid
flow field plate for current collection, for distributing oxygen to the second
face of the
cathode diffusion structure, and for extracting excess water from the MEA. A
plurality of
such fuel cells are conventionally layered in a series configuration to form a
fuel cell
stack.
A fuel cell stack may be conveniently disposed within a housing or other
supporting
frame or structure which may also provide support and/or protection for other
components of a fuel cell system. The other components of the fuel cell system
may
include various elements, such as one or more fans for forced ventilation of
the cathode
diffusion structure to deliver oxygen, a fuel delivery system, fuel and air
flow monitoring
systems, a temperature monitoring system, a cell voltage monitoring system,
and
electronics for providing control functions to the fuel cell stack.
One additional component commonly used in a fuel cell system is a hydrogen
detector
for detecting hydrogen leaks from the fuel cell stack or from a supporting
fuel delivery
system. A hydrogen leak detector can be an important feature for safe
operation of a
fuel cell stack but adds cost and complexity to the construction of the fuel
cell system.
Further, a hydrogen leak detector may successfully detect the presence of a
hydrogen
leak, but does not actively assist in dealing with escaped gas; rather the
leak detector is
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conventionally used to trigger an alarm condition and/or to shut down the fuel
cell and/or
fuel supply.
It is an object of the present invention to provide an improved system for
detecting
hydrogen leaks in a fuel cell system.
According to one aspect, the present invention provides a fuel cell system
comprising: a
fuel cell including at least one cathode and a cathode conduit for passage of
oxidant gas
through or over the cathode; a housing containing said fuel cell and defining
a plenum
around the fuel cell; a ventilation system configured to force air from the
plenum into the
cathode conduit; and a control system configured to monitor the fuel cell
voltage and to
detect a drop in voltage attributable to the presence of hydrogen in the
cathode conduit.
The housing may define a plenum confining all faces of the fuel cell except a
cathode
exhaust face thereof, the cathode exhaust face including a downstream end of
the
cathode conduit. The ventilation system may comprise a fan disposed in a wall
of the
housing configured to blow air into the plenum. The housing may be configured
such
that the primary exit path for air in the plenum is via the cathode conduit.
The system
may include a plurality of fuel cells formed into one or more stacks within
said housing.
The control system may comprise: a cell voltage monitoring system for
determining the
actual voltage of the fuel cell or of one or more cells in said fuel cell
stack; a processor
for receiving inputs indicative of the operating conditions of the fuel cell
or fuel cell stack
and determining therefrom an expected voltage of the one or more fuel cells
being
monitored; and a comparator for determining whether the difference between the
actual
voltage and the expected voltage exceeds a predetermined threshold indicative
of a
predetermined level of hydrogen in the cathode conduit. The housing may
comprise at
least one air inlet for passage of ambient air into the plenum, the system
further
including a fan disposed downstream of the cathode conduit and configured to
pull air
from the plenum into the cathode conduit and exhaust the air therefrom.
Embodiments of the present invention will now be described by way of example
and with
reference to the accompanying drawings in which:
Figure 1 shows a fuel cell system comprising a fuel cell stack disposed within
a
housing; and
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Figure 2 shows a schematic diagram of the functional components of a hydrogen
leak detection system incorporated into the fuel cell system of figure 1.
With reference to figure 1, a fuel cell system 1 comprises a fuel cell stack 2
disposed
within a housing 3. The housing 3 contains the fuel cell stack 2 and defines
an air space
or plenum 4 around the fuel cell stack. The fuel cell stack 2 includes a
plurality of cells 5
layered together in a conventional stack configuration so that the output
voltages of
successive cells 5 in the stack 2 can be coupled in series to provide a stack
output of
any desired voltage. The fuel cell stack 2 may be of conventional construction
to include
anode channels for delivering fuel to the anode sides of membrane-electrode
assemblies in the stack, and cathode channels for delivering oxidant to the
cathode
sides of the membrane-electrode assemblies in the stack. The fuel cell stack 2
may be
of the open cathode type such that oxidant is delivered to the cathode side of
the MEA
by forced ventilation at substantially atmospheric pressure through the
cathode
channels. The stack 2 comprises a number of faces including a cathode inlet
face 6
into which cathode air is blown and a cathode exhaust face 7 from which
cathode air is
expelled. The housing 3 preferably does not entirely enclose the fuel cell 2,
but leaves
the cathode exhaust face 7 exposed to ambient air on the exterior of the
housing 4 as
shown in figure 1. The housing may be sealed to the edges of the stack 2 to
prevent air
from passing around the edge of the stack.
The housing 3 includes an aperture 8 and an associated fan 9 therein which,
during
operation of the fuel cell stack, is configured to force air into the plenum 4
thereby
slightly raising the pressure of the air in the plenum so that it is forced
into the cathode
inlet face 6, passes through cathode conduits in the fuel cell stack 2 and is
expelled via
the cathode exhaust face 7. While the air is conveyed through the cathode
conduits, it
provides oxidant gas that passes through or over the cathode surfaces of the
fuel cells in
the stack, and carries the water by-product out of the cathode to the exhaust
face 7.
The fan 9 may also be positioned to blow the air over any supporting
electronics 15
within the housing, which serves to cool circuit devices and thereby to
preheat the air
passing into the fuel cell. Other forms of ventilation system to generate
appropriate air
flows may be used, e.g. blowers, compressors or the like. The expression "fan"
is
intended to encompass all such generators of air flows.
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Fuel cell stacks are typically made from tens or hundreds of layers including
membrane-
electrode assemblies, diffusion layers, fluid flow plates and gaskets sealing
the various
layers. Imperfect sealing of the layers, or degradation of materials used over
time, can
result in escape of hydrogen gas. Therefore, in many applications, it is
desirable to
monitor for escape of hydrogen gas from the stack 2. In the design of figure
1, it will be
recognised that an escape of hydrogen from the stack 2 will primarily result
in the
hydrogen leaking into the plenum 4. In addition, any hydrogen escape from a
supporting
fuel delivery system also contained in the housing 3 will result in the
hydrogen leaking
into the plenum 4. The inventors have recognised that it is not necessary to
provide a
separate hydrogen detector within the plenum 4 because the fuel cell stack
itself, in
conjunction with a suitable control system, may be used to detect any hydrogen
leak.
Any hydrogen leaking into the plenum 4 will be force-ventilated through the
cathode inlet
face 6 and into the cathode conduits of the fuel cell stack. Hydrogen present
at the
cathode face of the MEA of a fuel cell results in a drop in voltage and loss
of efficiency of
the fuel cell. This voltage drop can be detected by careful monitoring of the
fuel cell
using a control system as described below. In addition, a hydrogen leak in the
close
vicinity of the inlet to the housing, but not necessarily within the housing,
could also be
drawn in to the housing by the fan 9 and result in the hydrogen being force-
ventilated
through the cathode conduits.
As shown in figure 2, the fuel cell stack 2 includes at least two cell voltage
monitoring
terminals 20 which can be used to detect the voltage of a cell or a series of
cells or even
the whole stack. Fuel cells may have cell voltage monitoring terminals on many
cells in
the stack, or on selected groups of cells connected in series. The cell or
stack voltage(s)
is / are passed to a cell voltage monitoring circuit 21 and logged. A
processor 22
monitors the fuel cell voltage or voltages and is configured to detect a drop
in cell or
stack voltage attributable to the presence of hydrogen in the cathode conduit.
To do this, the processor 22 may operate to detect an unexpected change in
cell or
stack voltage output and trigger a detection condition in the event of an
unexpected fall
in cell or stack voltage. Alternatively, the processor may operate to compare
an actual
cell or stack voltage output with an expected cell or stack voltage output,
given prevailing
operating conditions for the cell or stack. To do this, the processor 22 may
be provided
with a plurality of inputs 23 corresponding to sensed operating conditions of
the fuel cell,
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cells or stack. These operating conditions may include such parameters as
temperature,
fuel flow, electrical load, cathode output humidity, local ambient humidity,
fuel cell age,
atmospheric air pressure, recent operational history etc. The processor may
use the
inputs to determine an operating condition which can be used to determine an
expected
voltage by way of an appropriate algorithm or look-up table. The processor may
include
a comparator for determining whether the difference between the actual voltage
measured and the expected voltage derived from the operating conditions of the
fuel cell
exceeds a certain amount or threshold that would be indicative of a
predetermined level
of hydrogen in the cathode conduit, e.g. arising from a hydrogen leak into the
plenum.
The extent to which the processor requires knowledge of some or all of the
operating
conditions identified above will depend on the sensitivity of hydrogen
detection required.
The cathode of a fuel cell can be extremely sensitive to the presence of
hydrogen such
that a substantial drop in the cell voltage, possibly even to near-zero, can
occur with only
a few parts per million (ppm) of hydrogen present in the cathode air flow.
A further benefit of the closed configuration of the housing 3 and ventilation
system of
figures 1 and 2 for hydrogen detection is that any leaking hydrogen that does
escape,
and that is detected by the fuel cell, will be to some extent depleted by the
reaction
taking place on the cathode side of the PEM. This will be exhausted safely as
water
from the cathode exhaust. Thus, not only can the leaking hydrogen be
conveniently
detected, but it can be partially or fully rendered harmless by the fuel cell
itself.
Various changes to the exemplary fuel cell system described in connection with
figures 1
and 2 could be made.
The cathode exhaust face 7 of the stack 2 could also be contained within the
housing 3
but be directly coupled to the exterior of the housing by a suitable
ventilation duct or
other air flow conduit.
The fan 9 could be positioned on or adjacent to the downstream cathode exhaust
face 7
of the fuel cell stack and be arranged to pull air from the plenum 4 via the
cathode
conduit. In such a configuration, the aperture 8 in the housing 3 would
preferably be
sized to ensure that the exhaust fan is strong enough to maintain a slight
negative
pressure in the plenum sufficient to ensure that sufficient hydrogen from any
significant
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escape will be drawn through the cathode conduit. The aperture 8 could
comprise a
number of apertures distributed around the housing for smoother air flow.
It is preferable that the housing 3 is relatively airtight to ensure that
hydrogen leaking
from the fuel cell stack 2 is captured within the housing and is then forced
into the
cathode conduits of the fuel cell. However, it will be understood that
complete
airtightness is not essential. The degree of closure of the housing required
will be
determined in part by such factors as the strength of the fan and the airflows
necessary
through the stack and the sensitivity of the stack to detecting small
concentrations of
hydrogen in a larger air flow. Thus, all that is required is that the housing
3 offers
sufficient containment to the plenum 4 to ensure that a sufficient proportion
of any
hydrogen leaking from the stack 2 is captured for forced ventilation through
the cathode
conduit for reliable detection. In a preferred arrangement, the housing and
ventilation
system is configured such that the primary (i.e. dominant) exit path for air
in the plenum
4 is via the cathode conduit.
The housing 3 can contain any size of fuel cell or fuel cell stack and
multiple stacks
could share a common housing.
Other embodiments are intentionally within the scope of the accompanying
claims.
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